The present invention relates to a device for controlling multiple hydraulic actuators, and, more particularly, to a device for controlling multiple hydraulic actuators to protect a system from an overpressure caused by increasing the load on an individual actuator.
The present invention can be applied, for example, to such a system as a power shovel that operates such multiple actuators as a spin motor, a boom cylinder, an arm cylinder, a bucket cylinder, and a driving motor.
In the prior art, described below with reference to FIG. 1, the drain pressure of a variable drain pump is controlled by the maximum load pressure of in any one of multiple actuators. The prior art has drawbacks as follows.
In the prior-art device of FIG. 1, when the variable orifice of one of a pair of selector valves sets a minimum value and the variable orifice of the other selector valve sets a maximum value, the cylinder attached to the other selector valve makes a full stroke, causing the load to increase. The increased load triggers an overload relief valves associated with that cylinder. Because the power (or Q.times.P, where Q is quantity of fluid and P is pressure) remains constant, the amount of fluid drained by the variable drain pump has to decrease, as shown by the curve in FIG. 2. Therefore the supply of fluid decreases to that selector valve set at the minimum value. In the worst case, the fluid supply drops so low that the cylinder stops altogether, thereby stopping all the actuators.